Today, advance of microelectronic industry also promotes memory technology to achieve an increased integration level and a reduced manufacture cost. Non-volatile memory devices maintain information data stored therein after power off and play an important role in information storage.
One of novel non-volatile memory devices, called resistive random access memory (RRAM) or resistive switching memory, uses a variable-resistance material and has the advantages of high operation speed (<5 ns), low power dissipation (<1V) and easily being integrated with other semiconductor devices. Such non-volatile memory devices are a prospective candidate for next-generation memory devices. The resistive switching memory device typically has an metal-insulator-metal (MIM) configuration in which a layer of variable-resistance material is sandwiched between two metal electrodes.
The variable-resistance material is typically a transition metal oxide, such as NiO, TiO2, HfO2, ZrO2, ZnO, or the like. The variable-resistance material can have two stable states, i.e. a high-resistance state and a low-resistance sate, which correspond to digital “0” and digital “1” respectively. A change from the high-resistance state to the low-resistance state is referred to as programming or SET operation, and a change from the low-resistance state to the high-resistance state is referred to as erasing or RESET operation.
The resistive switching memory devices can be classified as unipolar or bipolar devices according to its operation scheme. According to the former, a voltage is applied across a resistive switching memory device in one direction, and changes a resistance state of a variable-resistance material between a high-resistance state and a low-resistance with different amplitudes of the voltage to write data into the resistive switching memory device or erase data therefrom. According to the latter, a voltage is applied across the resistive switching memory device in two opposite directions for changing the resistance state of the variable-resistance material respectively. The bipolar resistive switching memory devices are superior to the unipolar resistive switching memory devices in memory performances such as switching speed, uniformity, reliability (data retention and endurance), controllability, or the like.
The resistive switching memory devices can be classified as 1T-1R configuration or 1D-1R configuration according to its basic configuration. Each memory cell in the 1T-1R configuration includes one select transistor and a resistive switching memory cell. By controlling the select transistor, data can be written into a selected memory cell or erased therefrom. Because the select transistor is used, it occupies a major portion of a footprint of the memory cell, which hinders an increase of the integration level. Each memory cell in 1D-1R configuration includes one diode and a resistive switching memory cell. The diode is used to suppress the sneak path current through the memory array. Because the diode typically occupies an area less than a transistor, the memory cell in 1D-1R configuration is advantageous in an increase of the integration level.
However, conventional resistive switching memory devices in 1D-1R configuration can only operate in unipolar resistive switching scheme due to limited material properties of the diode, which in turn limits memory performances.